McFerland's scale

I use McFarland turbidity scale to prepare bacterial cells suspension for my routine work. The turbidity scales are prepared in our lab itself by using BaCl2 and H2SO4. I face problem of poor stability of the BaSO4 precipitate in the scale and it forced us to prepare very often. Can anybody help me to stabilise this precipitate so that the prepared scales can be stored and used for long time.